Speaker
Florian Penzel
(Max Planck Institut für Plasmaphysik)
Description
The ITER bolometer diagnostic will have to provide accurate measurements of the plasma radiation in a varying thermal environment of up to 250°C. Current fusion experiments perform regular in-situ calibration of the detector properties, assuming stable calibration parameters within short discharge times, e.g. 10 s on ASDEX Upgrade. For long-pulse fusion experiments, e.g. W7-X, the diagnostic is operated with water cooling for achieving a stable temperature environment. However, ITER will be equipped with over a hundred bolometer cameras and is planned to have discharge times of up to 1 h. Due to space restrictions, active cooling is not available for all locations. Thus, an alternative approach is required to allow for compensation of the changing calibration values due to thermal drifts.
This paper demonstrates a method using the Wheatstone-bridge current of the detector to calculate in real-time the changing calibration values, such as the heat capacity, the thermal time constant and the meander resistances. It is shown, that the thermal offset error, a calibration parameter drift associated with the production tolerances between measurement and reference meander resistances, can be calculated by extrapolating from the initial spread of the resistances.
Measurements in the ITER Bolometer Vacuum test facility (IBOVAC), used to simulate ITER-relevant thermal and vacuum environment, show that the change of the calibration values can be predicted during repeated thermal cycles over durations sufficient for ITER discharges and even longer. Confidence intervals for each parameter of the in-situ calibration method are determined and compared with the accuracy of the proposed extrapolation method for ITER showing that this method provides an equivalent quality of the measurement results.
Co-authors
Daihong Zhang
(Max Planck Institut für Plasmaphysik, Greifswald, Wendelsteinstr. 1, Germany)
Florian Penzel
(Max Planck Institut für Plasmaphysik, Garching. Boltzmannstr. 2, Germany)
Hans Meister
(Max Planck Institut für Plasmaphysik, Garching. Boltzmannstr. 2, Germany)
Louis Giannone
(Max Planck Institut für Plasmaphysik, Garching. Boltzmannstr. 2, Germany)
